Easy? Nope, not really. First you will need a Windows XP license, that can run you up to $289, depending where you buy and what type you get.
Then you will need the drivers for Windows XP. There is a chance that the manufacturer won't even have them available. Since you didn't include the make and model of the laptop that you want to downgrade, I can't even begin to guess.
There is no real "down grade" you will need to boot from a Windows XP installation CD, delete the Vista partition, then recreate and format the new partition. Next you will need to install the drivers for all the hardware (e.g. video, sound card, network adapters, chip sets, etc.)
My recommendation is DON'T DO IT! Unless you are a computer geek and enjoy this stuff, it's not an easy task. Furthermore, Windows 7 is right around the corner and you can upgrade from Vista.
Patrick McHugh, October 2009